Hitherto, in FM-CW radars, serving as on-vehicle radars, that use a millimeter wave, there has been a problem of interference with radars mounted on other vehicles. More specifically, as shown in FIG. 4(A), when a vehicle MM and another vehicle OM1 on each of which a radar that radiates beams in an azimuth direction is mounted face each other, a spike noise is superposed on (mixed in) a beat signal at the time that the beat is caused between the reception signal and a transmission signal of the vehicle MM after the vehicle MM directly receives a transmission signal from the other vehicle OM1. In addition, as shown in FIG. 4(B), a vehicle MM on which a radar that performs beam scans is mounted and another vehicle OM2 on which a monopulse radar is mounted face each other, a spike noise is superposed on a beat signal of a transmission signal and a reception signal. Furthermore, as shown in FIG. 4(C), when another vehicle OM4 that runs in front of a vehicle MM exists and still another vehicle OM3 that transmits a radio wave to this other vehicle OM4 exists, a signal transmitted from the radar mounted on the other vehicle OM3 and reflected from the other vehicle OM4 is superposed on a reception signal of a radar of an vehicle MM, due to which a spike noise is superposed on a beat signal.
Patent Document 1 discloses a method for performing detection of such a spike noise.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-168947
In a method described in Patent Document 1, when an amplitude level of a reception signal exceeds a predetermined threshold, it is determined that interference exists.
However, there is a problem that the interference cannot be detected except when a spike noise having the amplitude greater than the expected maximum amplitude of a reflected signal (i.e., a reception signal) is mixed in.
In another particular circumstance, Patent Document 1 also describes that, when the frequency of a signal is higher than a predetermined value, the signal is considered as an interference wave. However, high frequency components unnecessary for target detection have only to be excluded from a subject of the target detection, which is not problematic from the beginning.
In addition, in still another particular circumstance, Patent Document 1 describes that, if a correlation between a beat signal determined in the last frame and a beat signal determined in this frame is not obtained in a case where one frame is constituted by an up-modulation interval and a down-modulation interval and the frame is repeated, it is determined that “interference exists”. However, when relative speed of a vehicle or another vehicle, namely, a target, is high, the correlation cannot be obtained and it may be mistakenly determined that “interference exists”.